Separate Analysis and Results.

`Results` contains and `Analysis` and an `EntryStates`. The unfortunate
thing about this is that the analysis needs to be mutable everywhere
(`&mut Analysis`) which forces the `Results` to be mutable everywhere,
even though `EntryStates` is immutable everywhere.

To fix this, this commit renames `Results` as `AnalysisAndResults`,
renames `EntryStates` as `Results`, and separates the analysis and
results as much as possible. (`AnalysisAndResults` doesn't get much use,
it's mostly there to facilitate method chaining of
`iterate_to_fixpoint`.)

`Results` is immutable everywhere, which:
- is a bit clearer on how the data is used,
- avoids an unnecessary clone of entry states in
  `locals_live_across_suspend_points`, and
- moves the results outside the `RefCell` in Formatter.

The commit also reformulates `ResultsHandle` as the generic `CowMut`,
which is simpler than `ResultsHandle` because it doesn't need the
`'tcx` lifetime and the trait bounds. It also which sits nicely
alongside the new use of `Cow` in `ResultsCursor`.

compiler/rustc_mir_dataflow/src/lib.rs

@@ -18,8 +18,8 @@ pub use self::drop_flag_effects::{
     move_path_children_matching, on_all_children_bits, on_lookup_result_bits,
 };
 pub use self::framework::{
-    Analysis, Backward, Direction, EntryStates, Forward, GenKill, JoinSemiLattice, MaybeReachable,
-    Results, ResultsCursor, ResultsVisitor, fmt, graphviz, lattice, visit_results,
+    Analysis, Backward, Direction, Forward, GenKill, JoinSemiLattice, MaybeReachable, Results,
+    ResultsCursor, ResultsVisitor, fmt, graphviz, lattice, visit_reachable_results, visit_results,
 };
 use self::move_paths::MoveData;